For certain applications, it is desired that artificial leather fabrics have a smooth surface, e.g., shoes having the appearance of calf-skin leather shoes. Artificial leather fabrics usually have a component comprised of fibers in some engaging association, e.g., woven, knitted, felted, needled, etc. This fibrous engagement limits, at least to some extent, certain physical properties of the fabric such as elongation, as opposed to an artificial leather made wholly of a polymeric film or film-like element.
Artificial leather fabrics have encountered some of the problems associated with tanned leathers, and this is not surprising, bearing in mind that leather is a fibrous material. One of these common problems is known as "orange peel." This problem is well known in the shoemaking arts and is of concern when leathers are lasted into shoes having a smooth surface. In the lasting process, leather is pulled over a shoe last to mold the leather to the three-dimensional surface of the last. This stretching often extends the leather by about 10% or slightly more on an area basis. For some particular styles of shoes and in some particular types of lasting operations, the leather may be extended, at least on a local basis, in excess of this percentage, i.e., about 15% or more. Fine calfskin leather can be stretched these amounts and still maintain a smooth surface after lasting. On the other hand, many other leathers, e.g., "split" leathers are incapable of maintaining a smooth surface with such area extensions and develop local non-planar areas on the surface. These non-planar areas are usually in the form of small depression, the size of which may be quite small in diameter, i.e., as little as 1/16th inch or less, or even a 32nd or 64th inch or less. Also, the depressions may extend into the surface of the leather a very small distance, e.g., a few thousandths of an inch or even less. Nevertheless, these depressions reflect light unevenly and produce an unsightly, undulated surface appearance. This is particularly true when light is reflected from a highly polished smooth leather surface. In some cases, the depressions can disfigure the surface of the leather in such a manner as to produce roughness which resembles the surface of an orange and the art describes those cases as "orange peel." The absence of "orange peel" is one of the indications of acceptable quality leather, or, alternately, the presence of "orange peel" indicates poor quality leather.
The mechanism responsible for these differences in leathers, e.g., between calfskin and "split" leathers, which allows or avoids "orange peel" is similar to fundamental mechanisms underlying the present invention. Thus, an understanding of "orange peel" in leathers elucidates important features of the present invention and is, therefore, explained below.
Animal skins which are tanned to produce leather are usually characterized as being composed of two layers, i.e., the epidermis and the dermis. The epidermis or surface is largely removed during the tanning process leaving the dermis which is in turn made up of two regions, commonly referred to as the papillary or grain layer and the reticular or corium layer. Both regions are composed of collagen fibers. The grain region is made up of short, fine, closely packed fiber bundles. The corium region is made up of longer, coarser and more loosely packed fiber bundles. In spite of its density, the grain layer is soft, particularly near the grain-corium junction. In this region the original animal skins contained fat cells, sweat glands, and hair follicles. Since these are removed in the course of converting the skins into leather, there remains a spongy or low modulus plane connecting the grain layer to the corium. This spongy region is very pronounced in leather made from the skin of certain animals, i.e., for example, sheep. While it is less pronounced in calfskin leather, it is nevertheless quite distinctly present. In the leather art, reference is made to the character of the bonding of the grain and reticular layers through this low modulus, spongy plane as "tight", "loose", or as having "many" or "few" bonding points.
The fiber network of the corium layer gives leather its strength and tough character. During stretching, the fiber bundles of this layer adjust to accept load and non-uniform, load-induced movement is inevitable in this layer. However, in calfskin leather the corium layer is relatively loosely connected through the low modulus, spongy layer to the grain layer and this loose connection tends to dissipate the load-induced, non-uniform movements and the surface of the grain layer remains smooth. On the other hand, in "split" leathers, this low modulus, spongy plane is removed during the "splitting" operation and these load-induced, irregular movements tend to transmit to the surface of the "split" leather. Under the effects of higher elongations, the irregular movements show up as "orange peel" in the "split" leathers. The same is true for other leathers where this low modulus, spongy plane, i.e., the loose connection, is either not naturally present or where it has been destroyed. Thus, "orange peel" is avoided in the leather arts simply by avoiding the use of such leathers, i.e., choosing high quality leathers, in lasting shoes in the manner described above. Unfortunately, in artificial leather fabrics, these problems cannot be so simply solved.
"Orange peel" has been a particularly difficult problem with artificial leather fabrics and the art has sought various solutions thereto. In one artificial leather, a tightly-woven high-count fabric was placed between a non-woven textile fabric base and a surface film. The intended function of this tightly-woven fabric was to control stretching of the artificial leather fabric during lasting into shoes and, hence, avoid local elongations which would induce "orange peel." Unfortunately, this tightly-woven base presented additional forming problems to the artificial leather, including elastic memory, and was not an acceptable arrangement. In aother artificial leather, a relatively thick foamed film was placed on the surface of a non-woven textile fabric so that irregular movements induced in the fibrous base during lasting of shoes were not fully transmitted through the thick foamed film to the surface thereof. While this approach does, indeed, eliminate most of the problem, it causes other problems, especially in that the soft film is easily scuffed, snagged or torn and does not produce a very durable product.
It will be appreciated from the foregoing that the undesired appearance of "orange peel" or surface roughness is a problem with all types of artificial leathers, but the problem is pronounced with smooth-surfaced artificial leathers. It is in this connection that the present invention has its major utility. In this regard, for purposes of the following disclosure and claims, the term "smooth surface artificial leather fabrics" refers to those artificial leather fabrics which have a visual appearance from a conversational distance similar to calfskin leathers, as opposed to heavily embossed leathers. However, even in heavily grained artifical leather fabrics, there may be areas between the heavy embossing where "orange peel" or surface roughness may constitute a problem. Therefore, the foregoing term is intended to include those grained artificial leather fabrics where "orange peel" or surface roughness constitute a problem.